As an optical fiber having low Rayleigh scattering loss and low transmission loss, a silica glass-based optical fiber whose core contains an alkali metal element or alkaline earth metal element is known (for example, refer to US 2005/0129376 A1 and U.S. Pat. No. 7,088,900 B1). Such an optical fiber is manufactured by drawing an optical fiber preform whose core portion contains an alkali metal element or alkaline earth metal element. In the case where the core portion of the optical fiber preform contains an alkali metal element or alkaline earth metal element, the viscosity of the core portion can be decreased during drawing of the optical fiber preform, and the network structure of silica glass can be made homogeneous. Therefore, the Rayleigh scattering loss resulting from inhomogeneity of the structure can be reduced. In this description, unless otherwise indicated, an alkali metal element or alkaline earth metal element will be generically referred to as an “alkali metal element or the like”.
A diffusion method is a known technique for adding an alkali metal element or the like into silica glass. In the diffusion method, while introducing raw material vapor of an alkali metal element or the like or a salt of an alkali metal or the like, which serves as a raw material, into a silica glass-based glass pipe, by heating the glass pipe with an external heat source or generating plasma inside the glass pipe, the alkali metal element or the like is diffused and added to the inner surface of the glass pipe.
After the alkali metal element or the like has been added to the inner surface and its vicinity of the glass pipe, the diameter of the glass pipe is reduced by heating. After the diameter reduction, for the purpose of removing transition metal elements, such as Ni and Fe, which are impurities simultaneously added during the addition of the alkali metal element or the like, the inner surface of the glass pipe is etched to a certain thickness. Since the alkali metal element or the like diffuses faster than the transition metal elements, the alkali metal element or the like is allowed to remain even when the transition metal elements are removed by etching the glass surface to a certain thickness. After the etching, the glass pipe is consolidated by heating, and thereby, a core rod containing the alkali metal element or the like in the central part thereof is produced. Glass that serves as an outer peripheral portion of the core may be added to the outer side of the core rod containing the alkali metal element or the like, and the whole body may be considered as a core portion of an optical fiber preform. A cladding portion having a lower refractive index than the core portion including the core rod containing the alkali metal element or the like is synthesized on the outer side of the core portion, and thereby, an optical fiber preform is produced. An optical fiber can be manufactured by melting the optical fiber preform in a high-temperature furnace, followed by drawing by a known method.